The present invention pertains to solid state quantum mechanical electron and hole wave devices and method for fabricating them and, in particular, to solid state quantum mechanical electron and hole wave devices such as, without limitation, low pass filters, high pass filters, narrow band and wide band notch filters, narrow band and wide band bandpass filters, impedance transformers, resonant electron and hole emitters, and so forth and method for fabricating them.
Recent progress in semiconductor growth technologies, particularly in molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD), enable those of ordinary skill in the art to grow multilayered superlattice structures with precise monolayer compositional control. For example, successively grown layers of narrow and wide band gap semiconductor materials such as GaAs and Ga.sub.1-x Al.sub.x As have been produced and widely used to provide multiple quantum well structures. In fact, there are many references in the prior art which are concerned with the use of these superlattice structures in resonant tunneling superlattice/multiple quantum well devices. Specifically, in such devices, a superlattice is formed by growing successive layers of narrow and wide band gap semiconductor material epitaxially and the materials and the widths of the layers in these devices are chosen so that quantum states which arise from spatial quantization effects in adjacent wells become coupled. Further, in such devices, the interaction of these coupled states leads to the formation of minibands of allowed energies through which carriers can tunnel.
Most of the above-described resonant tunneling superlattice devices disclosed in the prior art comprise a single quantum well, two barrier structure and such devices are of great interest as high frequency microwave oscillators. Recently, however, resonant tunneling through a multiple layer structure consisting of three wells and four barriers has been demonstrated in the GaAs/AlGaAs material system. Further, these structures have potential use as high energy injectors for electroluminescent devices, photodetectors, and fast ballistic transistors.
In addition to the above, there are prior art references which disclose the use of superlattices to provide miniband and forbidden energy bands at carrier energies above the barrier heights in order to produce negative differential resistance effects or to act as low-transmissivity blocking contacts.
In further addition to the above, there is presently great interest in the art in providing devices which exhibit high speed operation. Specifically, a major factor affecting the speed of semiconductor devices is the transit times of electrons from the input to the output terminals. If one can provide electrons which pass through the semiconductor without any scattering events, namely by "ballistic" or "collisionless" motion, then the transit time will be minimized and the potential speed of the devices will be maximized. The possibility of ballistic motion in semiconductor materials has recently been provided by experimental results in GaAs. As such, it is expected that when the length of the region to be traversed is on the same order as the electron mean free path (mfp), a sizable fraction of the electrons will traverse it ballistically. For example, although the mfp in silicon is on the order of 100 Angstroms (A), the mfp for electrons in GaAs is approximately 10 times greater.
In the interest of investigating the efficacy of fabricating such ballistic electron devices, experiments have been described in the prior art in which a GaAs layer is sandwiched between two layers of an alloy of AlGaAs. They report that AlGaAs is a suitable material for use therein because it has the same lattice constant as GaAs and, as a result, it can be grown epitaxially thereon. In addition, further reported experiments have shown that ballistic hole motion also occurs in GaAs, albeit at a lower fraction than that which occurs for electron motion due to the peculiar band structure of the valence band of GaAs.
As a consequence of the recent references available in the prior art, there exists a need for electron and/or hole filter devices, such as low pass, high pass, notch and bandpass filters which can be used to fabricate solid state devices requiring energy selectivity such as electroluminescent devices, photodetectors, ballistic transistors and so forth.